The invention is related generally to the field of data communication between wellbore measuring instruments and surface recording systems adapted thereto. More specifically, the invention is related to techniques for compressing data before encoding in a selected telemetry format, and decompressing the data to recover the most significant information contained in the originally measured data.
Many types of well logging instruments have been adapted to make measurements which can be converted into a visual representation or xe2x80x9cimagexe2x80x9d of the wall of a wellbore drilled through earth formations. Typical instruments for developing images include electrical resistivity measuring devices and acoustic reflectance/travel time measuring devices. These instruments measure a property of the earth formations proximate to the wall of the wellbore, or a related property, with respect to azimuthal direction, about a substantial portion of the circumference of the wellbore. The values of the property measured are correlated to both their depth position in the wellbore and to their azimuthal position with respect to some selected reference, such as geographic north or the gravitationally uppermost side of the wellbore. A visual representation is then developed by presenting the values, with respect to their depths and azimuthal orientations, for instance, using a color or gray tone which corresponds to the value of the measured property.
Wellbore images are typically developed using logging instruments that are lowered into and retrieved from the wellbore at one end of an electrical cable. The measurements made by the instrument, including value of the property being measured and measurements corresponding to the azimuthal orientation of the measurements, are transmitted to the recording and interpretation equipment at the earth""s surface. Other types of instruments, known as xe2x80x9clogging while drillingxe2x80x9d (LWD) instruments, are conveyed at the end of a drilling assembly used to drill the wellbore. An advantage of LWD instruments is that they measure properties of the formation during the drilling of the wellbore. Often the values of certain formation properties are of use to the wellbore operator when determined during drilling for purposes such as deciding at which depth to set a protective pipe or casing, steering the wellbore into the target formations, and determining whether the formations are likely to contain commercially useful quantities of hydrocarbons.
LWD instruments, in some cases, include a provision for sending at least some of the measurements made to recording equipment at the earth""s surface at the time the measurements are made using a telemetry system. One such telemetry system modulates the pressure of a drilling fluid pumped through the drilling assembly to drill the wellbore. The fluid pressure modulation telemetry systems known in the art, however, are limited to transmitting data at a rate of at most only a few bits per second. Because the volume of data measured by the typical image-generating well logging instrument is relatively large, at present, borehole images are generally available only using electrical cable-conveyed instruments, or after an LWD instrument is removed from the wellbore and the contents of an internal storage device, or memory, are retrieved.
One method known in the art for transmitting image-generating measurements in pressure modulation telemetry is described, for example, in U.S. Pat. No. 5,519,668 issued to Montaron. This method includes making resistivity measurements at preselected azimuthal orientations, and transmitting to the surface through the pressure modulation telemetry only the resistivity values. The method described in the Montaron ""668 patent requires synchronization of the resistivity measurements to known rotary orientations of the LWD instrument to be able to decode the image data at the surface without transmitting the corresponding rotary orientations at which the measurements were made.
Other data compression techniques, for various applications, are described in several other U.S. patents, for example, U.S. Pat. No. 5,757,852 to Jericevic et al; U.S. Pat. No. 5,684,693 to Li; U.S. Pat. No. 5,191,548 to Balkanski et al; U.S. Pat. No. 5,301,205 to Tsutsui et al; U.S. Pat. No. 5,388,209 to Akagiri; U.S. Pat. No. 5,453,844 to George et al; U.S. Pat. No. 5,610,657 to Zhang; and U.S. Pat. No. 6,049,632 to Cockshott et al. Most prior art data compression techniques for audio and video data compression and storage do not contemplate the extremely low bandwidth and very high noise level of the telemetry channel of the typical LWD pressure modulation telemetry system, and, have not been suitable for image transmission by such telemetry.
It is desirable to have a system which enables transmission of data for imaging a wellbore through pressure modulation telemetry so that images of a wellbore can be developed during the drilling of a wellbore, wherein the rotary orientation of each image-developing measurement is included in the transmitted data. It is also desirable to have a data compression system which facilitates measuring the rate at which rock is drilled and makes estimates of orientation of boundaries between layers of earth formations.
One aspect of the invention is a method for compressing a frame of data representing parameter values, a time at which each of the parameter values was recorded, and a corresponding azimuthal orientation at the time each of the parameter values was recorded. The method according to this aspect of the invention includes range compressing the parameter values. The method includes choosing a compression transforms that takes advantage of the natural periodicity of the data in the azimuthal direction, and then applying the transform in two-dimensions to the scale compressed parameter values, an output of the transform comprising a set of coefficients, and quantizing the coefficients.
One embodiment of this aspect of the invention includes a Fourier transform in the azimuthal domain and a discrete cosine transform in a domain corresponding to the time of recording. The corresponding domain can be the time itself or the depth domain, for example, where the term depth domain pertains to a measurement along the length of the wellbore.
In one embodiment of this aspect of the invention, the method further comprises encoding the quantized coefficients. One example of encoding includes efficient entropy encoding. In one embodiment, the method further comprises error-correction encoding the encoded quantized coefficients. One example of error-correction encoding includes interleaved encoding. One example of interleaved encoding comprises separating bits in a frame of data into words each having equal length, selecting correspondingly positioned bits in each of the words to form new words, and Hamming encoding the new words.
One embodiment of this aspect of the invention includes applying the Hamming encoded new words to a selected location in a telemetry sequence, and transmitting the Hamming encoded new words to a recording unit. Another aspect of the invention is a method for acquiring and communicating image developing to a surface recording unit. The method according to this aspect of the invention includes measuring a value of a parameter of an earth formation penetrated by a wellbore at azimuthally spaced apart positions in the wellbore, determining a rotary, or azimuthal, orientation at which each of the values of the parameter is measured, and determining a time at which each of the values of the parameter is measured. The parameter values are compressing and encoded, and the encoded, compressed parameter values are applied to a selected position in a telemetry format for transmission to the surface recording unit. In one embodiment, the compression includes range compression of the parameter values, such as by calculating a logarithm of each parameter value. The compression in this embodiment further includes calculating a Fourier transform of the parameter values in the azimuthal domain, and calculating a discrete cosine transform in the time domain. The transform coefficients in this example are quantized and encoded. In this embodiment, the encoding comprises efficient entropy encoding. One embodiment of this aspect of the invention includes error correction encoding, which in this embodiment is an interleaved encoding scheme, including separating bits in a frame of data into words each having equal length, selecting correspondingly positioned bits in each of the words to form new words, and Hamming encoding the new words.
One embodiment of this aspect of the invention includes applying the Hamming encoded new words to a selected location in a telemetry sequence, and transmitting the Hamming encoded new words to the surface recording unit.
Another aspect of the invention is a method for determining the rate of penetration of a drilling assembly in a wellbore as part of the compression process. This method includes measuring, near the drilling assembly, a property of earth formations penetrated by the drilling assembly. The measuring is performed by sensors at axially spaced apart locations on the drilling assembly. A time at which each of the measurements of the property are made is recorded. An azimuthal orientation of the sensors at the time each of the measurements is made is also recorded. Azimuthally corresponding ones of the measurements made at a first one of the spaced apart locations are correlated to measurements made at a second one of the spaced apart locations. It is then determined, from the correlating, when the first and the second one of the spaced apart locations have made measurements in substantially the same earth formation. The rate of penetration is calculated from a difference in time between the measurements made in the substantially same earth formation at the first and second spaced apart locations and from a distance between the first and second spaced apart locations.
One embodiment of this aspect of the invention includes correcting a time-depth record of the instrument made at the earth""s surface for the rate of penetration of the instrument thus determined.
Another embodiment of this aspect of the invention includes making the axially spaced apart measurements at a plurality of azimuthally spaced apart positions around the wellbore and determining a dip of earth formations penetrated by the wellbore by correlating the measurements made at azimuthally corresponding positions and calculating an axial displacement of formation boundaries with respect to the wellbore.
A third embodiment of this aspect of the invention included making the image-based correlation over time from the transformed data. This process can even be used to further compress image data measured by a plurality of sensors, by removing that part of the variation between the sensor measurements that is due to variation in the rate of penetration.
Another aspect of the invention is the derivation of additional parameters of interest as part of the compression process.
One embodiment of this aspect of the invention relates to the derivation of the azimuth of bedding planes intersected by the wellbore as part of the compression process. One embodiment of this compression scheme required the discrete cosine transform in the time domain together with choosing an azimuthal reference and taking a Fourier transform in azimuthal domain with respect to that reference. The output of the Fourier transform includes coefficients which are magnitudes of sine and cosine components in the azimuthal wavenumber domain. The size of the coefficients corresponding to the sine terms depends upon the azimuthal reference that is used to define the Fourier transform. An azimuthal reference which minimizes the size of the sine terms is precisely the azimuth about which the bedding planes will appear to be symmetrically positioned around the wellbore. This is the azimuth of the dipping plane. In this embodiment of the invention, the azimuthal reference which results in minimum size of the sine term coefficients is selected, and the corresponding azimuth of the dipping planes along with the sine terms, whose values will have been minimized by the process of subtracting the azimuth, are transmitted to the surface recording unit in LWD telemetry.
Other aspects and advantages of the invention will be apparent from the description which follows.